1. Field of the Invention
The present disclosure relates to renewable energy and more specifically to a solar position sensor apparatus and method.
2. Description of the Related Art
Solar energy collection systems that concentrate sunlight (e.g. for lighting, heating, or electricity production) must track the sun's relative motion during the day to optimize their efficiency. Many solar trackers use “closed loop” feedback control, where a solar position sensor is used to optically ascertain, usually through shadowing or imaging methods, the position of the sun relative to the concentrator. The output from this position sensor is then used to provide closed loop positioning feedback to adjust actuators to ensure that the concentrator is pointed at the sun.
Other solar trackers use “open loop” control and rely on solar position equations, such as those developed by Flandern and Pulkkinen, to calculate the position of the sun based on the latitude, longitude, date and time and control positioning actuators to ensure the concentrator is pointed at the sun without the need for any active solar position sensing.
One example of an open loop tracker is a Polar Aligned Single Axis Tracker (PASAT). This tracker uses a clock motor to rotate at the same angular rate as the rotation of the earth and is aligned so that its rotation axis is parallel to the axis of the earth. Neglecting gravitational effects of other celestial objects, it follows the apparent path of the sun throughout the course of the day. In the case of open loop solar trackers, a solar position sensor may still be used to correct for errors that may arise due to imperfections in the structure to which the tracker is mounted (e.g. being out of level, exhibiting self-weight deflection in various positions, gravitational effects of other celestial objects etc.).
Single axis trackers have one degree of freedom that acts as an axis of rotation. The axis of rotation of single axis trackers is typically aligned along a true North meridian. It is possible to align them in any cardinal direction with advanced tracking algorithms. There are several common implementations of single axis trackers. These include horizontal single axis trackers (HSAT), vertical single axis trackers (VSAT), tilted single axis trackers (TSAT) and polar aligned single axis trackers (PSAT). The orientation of the module with respect to the tracker axis is important when modeling performance.
Dual axis trackers have two degrees of freedom that act as axes of rotation. These axes are typically normal to one another. The axis that is fixed with respect to the ground can be considered a primary axis. The axis that is referenced to the primary axis can be considered a secondary axis. There are several common implementations of dual axis trackers. They are classified by the orientation of their primary axes with respect to the ground. Two common implementations are tip-tilt dual axis trackers (TTDAT) and azimuth-altitude dual axis trackers (AADAT). The orientation of the module with respect to the tracker axis is important when modeling performance. Dual axis trackers typically have modules oriented parallel to the secondary axis of rotation. Dual axis trackers allow for optimum solar energy levels due to their ability to follow the sun vertically and horizontally. No matter where the sun is in the sky, dual axis trackers are able to angle themselves to be in direct contact with the sun.
Unfortunately, one of the issues that arises with solar position sensors is the effect of haze and clouds on the fidelity of the position information. Most solar position sensors operate by locating the centroid of the sky's intensity, which, in a clear sky condition, would represent the sun. However, the effect of clouds and haze can cause that centroid to be displaced from the actual position of the sun. Examples are: an overcast sky with solar disk dimmed and surrounded by scattered light; a clear view of sun when it is surrounded by brightly reflecting clouds, and a small thin cloud directly in front of the sun producing large bright spot whose centroid may differ from the sun's actual position.
In the case of closed loop trackers, these effects will inevitably lead to pointing errors, as there is no a prior information about the sun's position to provide a secondary means of controlling the concentrator's position. In the case of open loop trackers, where the solar position information is used to correct for errors in the calculated solar position, the centroid errors arising from atmospheric scattering may be avoided by simply ignoring the data when it is deemed to be unreliable. Unfortunately, making that determination in an automated manner is challenging. Fairly sophisticated digital image processing algorithms may potentially be required to determine when the solar position information would be reliable enough to use for calibration points.
In order to effectively capture the maximum available solar energy from the sun, improvements to solar position sensors are needed.